In the field of offshore oil and gas exploration, in the early stages of developing a particular area or field, it is often necessary to first drill and complete test wells to determine the viability of continued exploration and drilling in that area. When conventional multi-legged fixed offshore platforms are installed during these early stages, the cost of the initial drilling and testing is greatly increased. This high initial cost reduces the incentive to speculate on drilling in untested locations and thus impedes the search for new oil and gas fields.
Efforts to reduce the cost of drilling and testing these initial exploration wells at unproven drilling locations have included supporting wellhead platforms from a single conductor pipe or main casing. This eliminates the necessity of having conventional platform installed from the onset of the drilling project. In the typical conductor supported installation, the main casing or conductor pipe supports a boat landing, ladders and a small platform to provide access to the wellhead. In shallow water, the main casing is mudline supported and must be sized to have sufficient structural integrity to be free standing above the mudline. In deeper water, the supporting of the main casing from the mudline becomes impractical and the main casing is supported by a system of cables attached to anchor piles driven into the water bottom around its periphery, much like a guyed tower or flag pole.
One method and apparatus for cable guyed offshore structures is described in U.S. Pat. No. 4,818,146 to Ozeman J. Fontenot. In Fontenot, the conductor pile is driven to refusal in the mudline below a body of water. An annular brace with an upper collar portion for attaching the ends of a plurality of support cables and a lower collar portion with a plurality of pulleys corresponding to the number of support cables being utilized is attached to the conductor pile. During installation one end of each cable is attached to an anchor pile. Each anchor pile is then releasably attached to a drive pile and anchor pile combination is driven by means of a hammer located and operated from a derrick barge above the water surface. When the anchor pile is driven to a satisfactory depth below the mudline beneath the water surface, the drive pile is released from the anchor pile and removed. The free end of each cable is then journalled around one of the pulleys on the lower collar portion of the brace, attached to the upper collar portion, and the cable is tightened by sliding the lower collar portion down the conductor pipe.
Another apparatus and method for installation of a cable supported main casing is described in U.S. Pat. Nos. 4,640,647 and 4,710,061 to Christon R. Blair and Kenneth B. Parker. The Blair et al patents disclose a method similar to that of Fontenot in that there is attached to the conductor pile longitudinally positionable clamping means having a plurality of pulleys for receiving the cables. When installing the Blair et al apparatus, anchor piles with an attached cable are driven with releasable drive piles in a method similar to that described in Fontenot. The free end of each cable is then positioned through its respective pulley on the clamping means, pulled tight and attached to the clamping means to provide support to the main conductor, also in a manner similar to that of Fontenot.
There are certain disadvantages to the methods and apparatus described by Fontenot and by Blair et al. First, the method of installing the anchor piles with the releasable drive piles at the desired distance and location around the caisson typically requires the use, and therefore the cost, of a derrick barge or other vessel from which to position the anchor pile and operate the pile driving hammer. This is in addition to the need for a drilling rig and dive boat to complete the other phases of installation.
Second, the use of pulleys positioned on the collar clamp to place tension on the cables to support the caisson has disadvantages. The pulleys are subject to corrosion due to the salt water environment which can reduce their effectiveness. The cables and the pulleys are also subject to metal fatigue at each cable-pulley interface from the repetitive stresses induced in the cable due to the wind, wave and tide actions found in offshore locations. This fatigue increases the chance of materials failure and results in the need for frequent inspection and maintenance of the pulleys, cables and clamps.
An additional disadvantage of both Fontenot and Blair et al is that the main conductor pipe must have a sufficient overall diameter and thickness to provide structural support for the ladders, the boat landing, the access platforms and the wellhead, even when the cables and anchor piles are in place to support the main conductor pipe. This necessarily requires the drilling contractor to use larger conductor pipe than that which might be required simply to serve as a conductor for the drilling and production tubing. The higher cost of the larger sized conductor pipe is reflected in the cost of drilling and testing the exploration wells.
Consequently, a need exist for improvements in the design and installation of cabled guyed offshore structures to decrease the cost of drilling and testing exploration wells at offshore locations.